Systematic Drilling and Blasting for Surface Excavations Part 6 docx

EM iii O-2-3800 1 Mar 72 available but hole, depths do not usually exceed iOO ft. Although some power augers can theoretically be utilized in the same rocks as those drilled with drag bits, their principal use has been in very soft rocks or in soil. 4-4. Percussive Drills. a. Percussive drills penetrate rock through the action of an im- pulsive blow thqough a chisel or wedge-shaped bit. Repeated applica- tion of large force of short duration crushes or fractuzes rock when the blow energy is of adequate magnitude. Torque, rotational speed, and thrust requirements are significantly lower for percussive systems than they are for rotary or rotary-percussive systems. Penetration rates in percussive drilling are proportional to the rate at which energy is sup- plied by the reciprocating piston. Q b. Percussive machines include churn drills, surface hammer drills, down-the-hole hammer drills, and fibratory drills. Surface ham- mer drills are those in which the hammer remains at the surface. Down-the-hole drills are those in which the hammer is near the bit within the hole. They are generally used for larger holes. Vibratory drills, still in the development stage, use a mechanical, electrical, or fluid- driven transducer to deliver a high- frequency, periodic force to the bit. c. Fig. 4-8 shows a small hammer drill. Several of me more common hammer bits and accompanying steel assemblies are shown in Figs. 4-9 and 4- iO. Each bit holds replaceable tungsten carbide in- serts. The bits are generally separate units detachable from drill steel. Hammer drills are capable of holes from i- i/2 to 5 in. in diameter. Hammer drills are extensively used for blasthole drilling. The most commonly used types and their general characteristics are detailed below. d. Jackhammers are hand-held, air- or gasoline-driven tools weighing from 37 to 57 lb. Air-driven models require between 60 and 80 cubic feet per minute (cfm) of air. Hole sizes range from i- 1/2 to 2 in., although larger drill bits are sometimes utilized in very soft rock. Jackhammers typically drill holes from 2 to 8 ft in depth and are sel- dom used to drill blastholes over 10 ft in depth. Stopers and drifters are larger hammer drills and were used originally in underground excavations. e. Wagon drills (usually mounted on rubber-tired wagons) have in the past been one of the more useful tools for rock excavations (Fig. 4-ii). Today, however, they are being replaced to a considerable 4-8 EM lfiO-2-3800 i Mar 72 AIR IN WATER @ HEXAGON @ / QUARTER OCTAGON @ ROUND SECTION B-B I Fig. 4-8. Ty-pical VE SECTION A-A o surface jackhammer drill design7 4-9 9 EM iiiO-2-3800 i Mar 72 SHIM TYPICAL ROCK KiRILL BITS ROTARY PERCUSSION 01T ( Fig. 4-9. Bits and steel assembly for surface ham- mer drills (figures show drive- on and threaded connections)9 ) TYPICAL STEEL AND SIT ASSEMBLY Fig. 4-i O. Bits for down- the- hole hammer dril19 4-10 EM il10-2-3800 i Mar 72 ,. degree by heavier,crawler drills. Wagon drills utilize 1- l/4-in. drill- ing steel, and bits range from f-3/4 to 3 in. in diameter. They are most effective at depths of less than 20 ft. They require between 275 and 300 cfm of air and, thus, can conveniently be paired with a 600-cfm compressor. 1 f. A single wagon drill can drill from 200 to 400 ft of hole in a 9-hr shift. The rate may be less in very hard rock such as granite. Considered another way, a single wagon drill can make blastholes to pro- duce be~een 500 and 1,500 cu . yd of rock per shift, depending L on the formation properties. At this average rate a contrac- tor would need three wagon drills to stay ahead of a 2- or 2- 1/2- yd shovel. g“ Crawler drills (Fig. 4- 12) have become tidely used \ -= - C“. = ~ d -k , u ) tools” in engineering exca-vation and have largely replaced the Fig. 4-ii. Wagon dri119 wagon drill. They are heavier units capable of drilling holes between 2- i/2 and 5 in. in diameter at any angle in all t~es of rock. These machines require about 50 percent more air, i.e. i50 cfm more than a wagon drill for a total of 450 cfm. Hole depths of 40 ft are routine and in some cases holes 100 ft in depth are put down svith heavy models. Crawler drills can produce blastholes resulting in as much as *O to three times more blasted rock per shift than wagon drills. h. The churn drill penetrates by repeatedly raising and dropping a hea~ chisel- shaped bit (Fig. 4-i3) and tool string at the end of a cable. The cuttings suspended in mud in the hole are periodically removed with a bailer. Churn drills are seldom used today in construction. 4-5. Rotary -Percussive Drills. a. Rotary-percussive drills impart three actions through the drill bit. These are (a) axial thrust, of lower magnitude than in rotary drill- ing, (b) torque of lower magnitude than in rotary drilling but higher than 4-i4 EM 1110-2-3800 i Mar 72 1 ,. J . Fig. 4-12. Crawler drill capable of drilling holes from 4-3/4 to 3 in. in diameter. Insets show setuD for vari- OuS hole inclinations ‘ 4-i2 L. r- WEAR SIJRFACE ANGLE OF CLEARANCE JL WATER COURSE — ANGLE OF PENETRATION EM ili O-2-3800 1 Mar 72 v Fig. 4-13. Churn drill bit9 4-13 ,1 EM li10-2-3800 i Mar 72 in percussive drilling, and (c) impact. Some drills hav,e a rotation mechanism that is actuated by the impact mechanism; whereas others have a separate motor to achieve rotation. The mechanism of rock failure may be considered as a combination of the rotary and percus- sive mechanisms. b. Drill bits such as those shown in Fig. 4-i4 have been success- fully used to drill deep blastholes from 4 to 9 in. in diameter. Conven- tional drill steel is used with down- the-hole drills, and since cuttings are removed up the annulus by air pressure, an air return velocity of around 50 fps is required. This velocity can be obtained with air sup- plies of around 15 cfm per in. of hole diameter in blastholes of moderate depth. ANVIL+ SHOULDER T / EIT GAGE A (w10E5T DIAM) PIN- MA PL AIR NOZZLE I (Courtesy of Colorado School of Mines) Fig. 4-14. i3 Rotary- percussive drill bit (after Liljestrand ) 4- i4 EM iiiO-2-3800 I 1 Mar 72 CWPTER,5. BASIC SURFACE BMTING TECHNIQUES 5-1. Introduction. a. Rock blasting may be conducted for removal of rock, for con- trol of excavated rock surfaces, and for control of blasted rock sizes. One project may require all types of blasting. For example, the con- struction of a large dam often requires removal of millions of cubic yards of overburden and rock, some of which may be wasted but much of which must be used for fill, riprap, and aggregate. Foundations, penstocks, and spillway walls should be excavated with controlled blasting to leave competent final surfaces. b. This chapter describes preferred blasting techniques used for surface excavations. Information was obtained from CE District offices and projects supplemented in less familiar procedures by references 8 and 14. 5-2. Blasting Patterns. a. Hole Arrays. (1) Hole array is the arrangement of blastholes both in plan and section. The basic blasthole arrays in plan are single-row, square, rectangular, and staggered arrays (Fig. 5- i). Irregular arrays have also been used to take in irregular areas at the edge of a regu- lar array. The term “spacing” denotes the lateral distance on centers between holes in a row. The “burden” is the distance from a single row to the face of the excavation, or betieen rows in the usual case where rows are fired in sequence. (2) Blasthole arrays in profile have characteristic hole depths and inclination. Fig. 5-2 shows how this geometry can vary. Deep and shallow holes are sometimes alternated to achieve particular results. Arrays using single holes are also used (Fig. 5-3). ● e ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● RECTANGULAR PATTERN ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● STAGGERED PATTERN ● ● ● ● ● SINGLE ROW Fig. 5-i. Basic blast- hole arrays 5-i EM iliO-2-3800 i Mar 72 I T —. -, I Fig. 5-2. Variation of regular arrangement of pro- duction blastholes as necessitated b~ topography 9 +e EXPLOSIVE ~’b \\ < STEMMING \< \ ‘\ \- ~ ADIT a. COYOTE TUNNEL (PLAN VIEW) ALSO SEE FIG. 5-15 c. POINT CHARGE b. SNAKE HOLE (PROFILE VIEW) Fig. 5-3. Single-hole arrays 5-2 EM ili O-2-3800 i Mar 72 b. Delay Pat~erns. The order of firing charges in a round is determined by the delay sequence , which is regulated by either a delay electric blasting cap or a delay detonating cord connector (Chapter 3). By varying delays modified as an aid vibration control. single-row,-square, and staggered patterns can be in achieving fragmentation, throw, rock removal, or Fig. 5-4 illustrates some possible delay patterns. . k, / o. EDGE OF BE NCI-I \ \ /- 5. ‘%- -+ + /’ ●s c. EOGE OF BENCH b. EDGE OF BENCH / Fig. 5-4. Some possible delay patterns: a-c, with electric de- lays; d, with detonating cord connectors. x indicates position of detonating cord delay connector. Numbers indicate firing order c. Arrangement of Charge in Hole. (1) Blasting agents and explosives may be placed in the hole in solid columns or in decked columns, i.e. with segments of the charges separated by stemming. Free-running ANFO is poured into the hole on top of a primer. Additional primers are commonly placed in the column at 10- to 20-ft intervals. The charge is detonated with either electric caps inserted in each primer or with detonating cord down line tied in contact with each primer. In large holes the charge may be efficiently detonated by initiating only the bottom primer with detonat- ing cord or blasting cap. Waterprod explosives or slurry blasting 5-3 . leave competent final surfaces. b. This chapter describes preferred blasting techniques used for surface excavations. Information was obtained from CE District offices and projects supplemented. Liljestrand ) 4- i4 EM iiiO-2-3800 I 1 Mar 72 CWPTER,5. BASIC SURFACE BMTING TECHNIQUES 5-1. Introduction. a. Rock blasting may be conducted for removal of rock, for con- trol of excavated rock surfaces,. overburden and rock, some of which may be wasted but much of which must be used for fill, riprap, and aggregate. Foundations, penstocks, and spillway walls should be excavated with controlled blasting